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Exercise about mol

  1. Oct 18, 2012 #1
    Hi everybody,

    sorry for my sloppy english. I have to translate the task so if you don't understand something just tell me and I try to rephrase it.

    1. The problem statement, all variables and given/known data

    The exersice is about the "Knallgas" equation. Two reaction euqations are given:

    [tex] 2H + O \xrightarrow \ 2H_2O +\Delta E[/tex]
    and
    [tex] 2H_{2} + O_{2} \xrightarrow \ 2H_{2}O +2\Delta E[/tex]

    The energy which is released in this equation is: dE=285.8kJ

    a) Decide which reaction equation is the right one. The decision should be based on experimental obseravtions.

    b) How much grams oxygen is needed for an optimal reaction when I have 0.0038 cm³ hydrogen?

    c) How much energy is released? How long could someone use that energy to run a hairdryer with it (P=1,2 kW)



    2. Relevant equations

    -

    3. The attempt at a solution

    a) Well, I think I can discuss that problem using the theorem from Avogrado. The ratio is the same in both equations. But in my opinion the task is missing a value for the volume of the water. But I say that the 2nd equation is right (That's what someone would find in the experiment)

    b) I searched for the density of H2

    [tex] \rho \cdot V=m [/tex]

    I get m=3.42*10^-4 kg
    also
    [tex] \frac {m(H_2)} {m(O_2)}= \frac 1 8[/tex]

    Now I just have to multiply the mass of H2 times 8 in order to get the mass from O2. I get m(O2)=2.73*10^-3kg


    c) I use the law of conservation of mass. The whole mass of water equals m(H2)+m(O2). I searched for the molar mass of water in order to calculate how much mol I have. Afterwards I calculate the energy the following way:

    E= E(given)*mol

    and for the last task I used the formula: P=dE/dt



    Is my solution right? Are there other possibilities to solve it? I'm uncertain because I had to look up some stuff like molecular weight, density etc. But I don't know if there's another way


    Thank you for your help in advance
     
  2. jcsd
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